Since the introduction of the first implantable pacemakers in the 1960's, there have been considerable advancements in both the fields of electronics and medicine, such that there is now a wide assortment of commercially available body-implantable electronic medical devices. This class of implantable medical devices (IMDs) generally includes therapeutic and diagnostic devices, such as pacemakers, cardioverter/defibrillators, hemodynamic monitors, neural stimulators, and drug administering devices, as well as other devices for alleviating the adverse effects of various health ailments.
As is known, modern electrical therapeutic and diagnostic devices for the heart and/or other areas of the body generally include an electrical connection between the device and a patient's body. This connection is usually provided by at least one medical electrical lead, which is typically implanted (at least partially) within the patient's body. For example, a neural stimulator delivers mild electrical impulses to neural tissue using one or more electrical leads. Such neural stimulation often results in pain relief or a reduction in tremors depending on where the electrodes are placed. Each electrical lead used with such devices typically takes the form of a long, generally straight, flexible, insulated set of conductors. At its proximal end, the lead is typically connected to a connector of the device, which also may be implanted within the patient's body. Generally, one or more electrodes are located at or near the distal end of the lead and are attached to, or otherwise come in contact with, the patient's body. Such devices may be controlled by a physician or the patient through the use of an external programmer.
Other advancements in medical technology have led to improved imaging technologies, e.g., magnetic resonance imaging (MRI). As further described below with respect to its process, MRI is an anatomical imaging tool which utilizes non-ionizing radiation (i.e., no x-rays or gamma rays) and provides a non-invasive method for the examination of internal structure and function. In particular, MRI permits 3-D imaging of soft tissue better than any other imaging method. During the MRI imaging sequence, a radio-frequency field is applied to the patient. Magnetic resonance spectroscopic imaging (MRSI) systems are also known and are herein intended to be included within the terminology “MRI” systems or scanners.
Further, shortwave diathermy, microwave diathermy, ultrasound diathermy, and the like have been shown to provide therapeutic benefits to patients, such as to relieve pain, stiffness, and muscle spasms; to reduce joint contractures; to reduce swelling and pain after surgery; to promote wound healing; and the like. Generally, in using such diathermy apparatuses, energy (e.g., short-wave energy, microwave energy, ultrasound energy, or the like) is directed into a localized area of the patient's body.
Traditionally, use of the above-described technologies have been discouraged for patients having IMDs, as the environment produced by the MRI or diathermy apparatuses is generally considered hostile to such IMDs. As is known, the energy fields, generated during the MRI or diathermy processes, have potential for inducing an electrical current within leads of IMDs as well as leads of other medical devices located within the patient. In conventional leads, this electrical current is typically conducted into tissue adjacent to the ends of the lead. Because the tissue area adjacent to the electrodes is often very small, the current conducting through this adjacent tissue results in the tissue heating. This may result in tissue damage when the currents are too large.
Thus, what are needed are medical device electrical lead systems that reduce tissue heating to levels that do not induce tissue damage.